1. Field of the Invention
The present invention relates to an image sensing apparatus and control method thereof and, more particularly, to a technique of correcting an image distortion which is caused by hand shake and contained in a sensed image.
2. Description of the Related Art
Recently, image sensing apparatuses such as a digital video camera popularly adopt, as an image sensor, a CMOS sensor lower in power consumption than a CCD sensor. The CCD sensor and CMOS sensor are different not only in power consumption but also in the exposure method in shooting.
In the CCD sensor, the exposure timing and exposure period are the same for all the pixels of the image sensor when sensing one image. To the contrary, in the CMOS sensor, the exposure period changes because the shutter opening/closing timing differs between lines of the image sensor. The driving method of the CMOS sensor is called a rolling shutter method.
Upon shooting with the rolling shutter type image sensor, the sensed image may distort because the object image moves for each line of the image sensor owing to movement of the object during exposure or hand shake of the user who grips the image sensing apparatus during exposure. The phenomenon in which distortion occurs is called the “rolling shutter effect” or “focal-plane effect”. Especially, a distortion caused by the rolling shutter effect may appear more conspicuously in moving image shooting using no mechanical shutter than in still image shooting using a mechanical shutter.
Japanese Patent Laid-Open No. 2006-186481 discloses a technique of correcting a distortion caused by hand shake of the user out of distortions arising from the rolling shutter effect. In Japanese Patent Laid-Open No. 2006-186481, the temporal characteristic of hand shake is obtained by integrating a hand shake speed detected by an angular velocity sensor arranged in the image sensing apparatus. The distortion is corrected in accordance with a positional change of the image sensing apparatus in shooting for each line of the image sensor.
However, the sampling frequency of the angular velocity sensor is several kHz to several ten kHz, while the hand shake frequency is about 0 Hz to 15 Hz. These frequencies are greatly different in order. When signal processing is performed using an FIR (Finite Impulse Response) filter described in Japanese Patent Laid-Open No. 2006-186481 to obtain the temporal characteristic of hand shake from the output signal of the angular velocity sensor, many taps are necessary, increasing the circuit scale.
To avoid the increase in circuit scale, not the FIR filter but an IIR (Infinite Impulse Response) filter may be used. However, the IIR filter does not have a linear phase, and a different phase lag or phase lead occurs in each frequency band. The temporal characteristic of hand shake obtained using the IIR filter has a phase shift depending on the phase characteristic of the IIR filter. The correction amount for correcting a distortion caused by the rolling shutter effect may not be accurately calculated for some hand shake frequencies. That is, when a distortion caused by the rolling shutter effect is corrected using a correction amount that is calculated from the temporal characteristic of hand shake obtained using the IIR filter, undercorrection or overcorrection may occur in the image.